Method of preparation of a MWCNT/polymer composite having electromagnetic interference shielding effectiveness

ABSTRACT

A method of preparing carbon nanotube/polymer composite having electromagnetic interference (EMI) shielding effectiveness is disclosed, which includes: dispersing multi-walled carbon nanotubes (MWCNT) in an organic solvent such as N,N-Dimethylacetamide (DMAc); dissolving monomers such as methyl methacrylate (MMA) and an initiator such as 2,2-azobisisobutyronitrile (AIBN) in the MWCNT dispersion; and polymerizing the monomers in the resulting mixture at an elevated temperature such as 120° C. to form a MWCNT/PMMA composite. The composite is coated onto a PET film, and the coated PET film alone or a stack of multiple coated PET films can be applied as an EMI shielding material.

This application is a divisional application of U.S. patent applicationSer. No. 12/081,517, filed Apr. 17, 2008, now U.S. Pat. No. 7,955,654,issued Jun. 7, 2011, and claims priority under 35 U.S.C. §119(e) toTaiwan Application Serial Number 96139396, filed Oct. 19, 2007, theentire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention provides a MWCNT/Polymer composite havingelectromagnetic interference shielding effectiveness and a method ofpreparation thereof.

BACKGROUND OF THE INVENTION

USP 2007/012900 A1 discloses a particulate conductive filler whichcomprises a conductive metal coating formed over a coarse carbon-basedcore such as graphite between 350 and 1000 microns in size. Theconductive filler is used in conjunction with a polymer matrix such asan elastomer typified by silicone elastomer to form composite materialsfor conductive and electromagnetic interference shielding applications.

WO 2007/010517 A1 discloses modified polymers which are prepared byproviding a nanotube or nanoparticle suspension, adding a preformedpolymer, swelling the preformed polymer in the suspension, and isolatingthe modified polymer from the suspension. The polymer may be a swellablepolymer in the form of polymeric yarns, fibres, fabrics, ribbons orfilms. The swelling may be carried out using ultrasonic treatment.Carbon nanotubes, magnetic (Fe3O4) and fluorescent (CdTe) nanoparticlessuspensions have been utilized to demonstrate the fabrication of newpolymer composites. The magnetic polymer composites are useful inelectromagnetic interference (EMI) shielding of medical equipment inhospitals, computers and consumer electronics.

USP 2005/127329 A1 discloses a method of reinforcing a polymericmaterial with nanosize materials, in which materials such as vapor growncarbon nanofibers, carbon nanotubes, layered silicates, nanosize spheredsilica, or graphite nanoparticles are combined with a polymer and asolvent to form a substantially homogeneous mixture, followed by removalof the solvent by evaporation or coagulation. Depending on the nanosizematerials used, the resulting polymeric nanocomposite material exhibitshigh electrical and thermal conductivity, enhanced mechanical strength,abrasion resistance, reduced gas permeation, and/or dimensionalstability. The polymeric nanocomposite material may be used inelectromagnetic interference shielding. The polymer used in this priorart invention is preferably selected from the group consisting ofpolyurethanes, polyolefins, polyamides, polyimides, epoxy resins,silicone resins, polycarbonate resins, acrylic resins, andaromatic-heterocyclic rigid-rod and ladder polymers.

WO 2004/097853 A1 discloses a conductive carbon nanotube-polymercomposite comprising carbon nanotubes and a polymer, wherein the carbonnanotubes primarily reside between coalesced particles of the polymer.The composite is prepared with a suspension of carbon nanotubes that canbe stabilized with a stabilizer, such as a water-soluble polymer orsurfactant. The nanotube suspension is mixed with a polymer suspensionof polymer particles that substantially exclude the nanotubes.

The polymer suspension can be stabilized with a stabilizer, such as awater-soluble polymer or surfactant. After mixing the two suspensions,water and any solvent are removed to form a nanotube-polymer composite.As liquid is removed from the nanotube-polymer suspension, the polymerparticles coalesce and the nanotubes become trapped and aggregateprimarily between the polymer particles, wherein the nanotubes form aconductive network in the polymer composite. Electrical percolation wasrealized with less than 0.04 wt % single-wall carbon nanotubes inpoly(vinyl acetate).

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a method ofpreparing carbon nanotube/polymer composite having electromagneticinterference (EMI) shielding effectiveness.

In order to accomplish the aforesaid objective a method of preparingcarbon nanotube/polymer composite having electromagnetic interference(EMI) shielding effectiveness provided in the present inventioncomprises the following steps: a) preparing a polymer solutioncontaining 0.5-10 wt % of carbon nanotubes dispersed therein, based onthe weight of the polymer; and b) coating the polymer solutioncontaining carbon nanotubes dispersed therein on a substrate and dryingthe resulting layer coated on the substrate.

Preferably, the method of the present invention further comprises thefollowing step: c) stacking a plurality of the substrates prepared fromstep b), each of which has the dried layer. More preferably, step c)further comprises applying an adhesive on the substrates prior to saidstacking so that the stacked substrates are bonded.

Preferably, the dried layer on the substrate prepared in step b) has athickness of 0.05 mm to 1.0 mm. More preferably, 2 to 100 sheets of thesubstrates are stacked in step c).

Preferably, said preparing in step a) comprises dispersing carbonnanotubes in an organic solvent; dissolving monomers and an initiator inthe carbon nanotube dispersion; and polymerizing the monomers in theresulting mixture to form said polymer solution containing carbonnanotubes dispersed therein. Said monomers preferably are selected fromthe group consisting of acrylic acid, methacrylic acid, methyl acrylate,methyl methacrylate, and styrene; or said monomers are a mixture ofacrylonitrile, butadiene and styrene. Among them, methyl methacrylate(MMA) is more preferable. Said organic solvent preferably isN,N-Dimethylacetamide (DMAc), said initiator preferably is2,2-azobisisobutyronitrile (AIBN), and said monomers preferably arepolymerized at 120° C.

Alternatively, said preparing in step a) comprises dissolving a polymerin an organic solvent and dispersing carbon nanotubes in the resultingpolymer solution to form said polymer solution containing carbonnanotubes dispersed therein. Said polymer preferably is selected fromthe group consisting of poly(acrylic acid), poly(methacrylic acid),poly(methyl acrylate), poly(methyl methacrylate), soluble polyimide,soluble poly(amide imide), polyamide, polystyrene, soluble polyurethane,unsaturated polyester, poly(ether sulfone), soluble poly(ether imide),poly(vinyl ester), thermoplastic polyurethane, silicone, and epoxyresin. Among them poly(methyl methacrylate) (PMMA) is more preferable.Said PMMA is preferably prepared by polymerizing methyl methacrylate ina solvent of N,N-Dimethylacetamide and in the presence of an initiatorof 2,2-azobisisobutyronitrile at 120° C.

Preferably, the carbon nanotubes are multi-walled carbon nanotubes(MWCNT).

Preferably, the carbon nanotubes are single-wall carbon nanotubes.

Preferably, the carbon nanotubes are double-wall carbon nanotubes.

Preferably, the carbon nanotubes are bamboo-type carbon nanotubes.

Preferably, the carbon nanotubes are spiral-type carbon nanotubes.

Preferably, the carbon nanotubes are TiO₂-coated carbon nanotubes.

Preferably, the carbon nanotubes are metal-coated carbon nanotubes.

Preferably, said substrate in step b) is a film of poly(ethyleneterephthalate), polyimide, polyethylene, polypropylene, or poly(vinylchloride).

Preferably, said substrate in step b) is a poly(ethylene terephthalate)(PET) film.

Preferably, said substrate in step b) is a polypropylene film.

Preferably, said substrate in step b) is a poly(vinyl chloride) film.

The present invention also provides a method of using a carbonnanotube/polymer composite to provide electromagnetic interference (EMI)shielding, which comprises preparing a carbon nanotube/polymer compositeby using the method of the present application as described above; andenclosing an electronic device in the carbon nanotube/polymer composite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot showing an effect of MWCNT content on the EMI shieldingeffectiveness of 1.0 mm MWCNT/PMMA composite sheets prepared in ControlExamples 1 to 6.

FIG. 2 is a plot showing an effect of MWCNT content on the EMI shieldingeffectiveness of 1.0 mm MWCNT/PMMA composite sheets prepared in ControlExamples 7 to 12.

FIG. 3 is a plot showing an effect of number of layers of 0.1 mmMWCNT/PMMA coated PET film on the EMI shielding effectiveness preparedin Examples 1 to 10 of the present invention.

FIG. 4 is a plot showing an effect of number of layers of 0.1 mmMWCNT/PMMA coated PET film on the EMI shielding effectiveness preparedin Examples 11 to 20 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A carbon nanotube/polymer composite having electromagnetic interference(EMI) shielding effectiveness prepared by a method according to one ofthe preferred embodiments of the present invention comprises thefollowing steps:

-   a) Dispersing MWCNTs in DMAc;-   b) Adding MMA and AIBN to the MWCNT dispersion prepared in Step (a);-   c) Polymerizing MMA at 120° C. to form a solution of MWCNT/PMMA    composite;-   d) Coating the solution from Step c) on a PET film and drying the    solution coated on the PET film; and-   e) Stacking a plurality of coated PET films from Step d) and using    the stacked PET films as an EMI shielding material.

A carbon nanotube/polymer composite having electromagnetic interference(EMI) shielding effectiveness prepared by a method according to anotherpreferred embodiment of the present invention comprises the followingsteps:

-   A) Dissolving MMA and AIBN in DMAc;-   B) Polymerizing MMA at 120° C. to form a solution of PMMA;-   C) Adding MWCNTs to the PMMA solution prepared in Step (B) and    dispersing MWCNTs in the PMMA solution under ultrasonication;-   D) Coating the PMMA solution containing MWCNTs dispersed therein    from Step C) on a PET film and drying the solution coated on the PET    film; and-   E) Stacking a plurality of coated PET films from Step d) and using    the stacked PET films as an EMI shielding material.

The present invention will be better understood through the followingExamples, which are merely for illustration and not for limiting thescope of the present invention. Materials used in the following Examplesand Control Examples were:

-   Multi-Walled CNT (abbreviated as MWCNT) produced by The CNT Company,    Inchon, Korea. This type of CNT was prepared by a CVD process. The    CNTs had a purity of 93%, a diameter of 10-50 nm, a length of 1-25    μm, and a specific surface area of 150-250 m² g⁻¹.-   Methyl methacrylate (MMA) manufactured by Acros Organics Co., New    Jersey, USA.-   2,2-azobisisobutyronitrile (AIBN) manufactured by Tokyo Chemical    Industry Co., Ltd., Tokyo, Japan.

CONTROL EXAMPLE 1

2.62 g of MWCNTs were dispersed in 97.5 g of DMAc; 52.5 g of MMA and0.11 g of AIBN initiator were added into the resulting dispersion; next,the mixture was allowed to undergo polymerization at 120° C., so that asolution of MWCNT/PMMA composite was formed. The MWCNT/PMMA compositesolution was used to form a sheet of 20 cm×20 cm×0.1 cm by molding.

CONTROL EXAMPLES 2-6:

The procedures in Control Example 1 were repeated except that the MWCNTcontent in the composite was varied as follows:

0.13 g of MWCNT was used in Control Example 2;

0.26 g of MWCNT was used in Control Example 3;

0.39 g of MWCNT was used in Control Example 4;

0.53 g of MWCNT was used in Control Example 5; and

131 g of MWCNT was used in Control Example 6.

CONTROL EXAMPLE 7

52.5 g of MMA and 0.11 g of AIBN initiator were dissolved in 97.5 g ofDMAc, and the solution was allowed to undergo polymerization at 120° C.to form a PMMA solution. 2.62 g of MWCNTs was dispersed in the PMMAsolution under ultrasonication. The resulting MWCNT/PMMA compositedispersion was used to form a sheet of 20 cm×20 cm×0.1 cm by molding,

CONTROL EXAMPLES 8-12

The procedures in Control Example 7 were repeated except that the MWCNTcontent in the composite was varied as follows:

0.13 g of MWCNT was used in Control Example 8;

0.26 g of MWCNT was used in Control Example 9;

0.39 g of MWCNT was used in Control Example 10;

0.53 g of MWCNT was used in Control Example 11; and

1.31 g of MWCNT was used in Control Example 12.

EXAMPLES 1-10

2.62 g of MWCNTs were dispersed in 97.5 g of DMAc; 52.5 g of MMA and0.11 g of AIBN initiator were added into the resulting dispersion; next,the mixture was allowed to undergo polymerization at 120° C., so that asolution of MWCNT/PMMA composite was formed. The MWCNT/PMMA compositesolution was coated on a PET film, and the resulting coating layer wasdried by evaporating the organic solvent therefrom to form a MWCNT/PMMAcomposite layer having a thickness of 0.1 cm on the PET film. Thecoating area was 20 cm×20 cm. 10 sheets of the coated PET films wereprepared as above. A single sheet of the coated PET film was used as anEMI shielding material, or multiple sheets of the coated PET films werestacked and the resulting stack was used as an EMI shielding material.

EXAMPLES 11-20

52.5 g of MMA and 0.11 g of AIBN initiator were dissolved in 97.5 g ofDMAc, and the solution was allowed to undergo polymerization at 120° C.to form a PMMA solution. 2.62 g of MWCNTs was dispersed in the PMMAsolution under ultrasonication. The resulting MWCNT/PMMA compositedispersion was coated on a PET film, and the resulting coating layer wasdried by evaporating the organic solvent therefrom to form a MWCNT/PMMAcomposite layer having a thickness of 0.1 cm on the PET film. Thecoating area was 20 cm×20 cm. 10 sheets of the coated PET films wereprepared as above. A single sheet of the coated PET film was used as anEMI shielding material, or multiple sheets of the coated PET films werestacked and the resulting stack was used as an EMI shielding material.

EMI shielding effectiveness (2-18 GHz) was measured by using a HP 8722ESVector Network Analyzer manufactured by Damaskos, Inc., Concordville,Pa., USA.

Results:

Table 1 shows the EMI shielding effectiveness at 15 GHz of theMWCNT/PMMA composite sheets prepared in Control Examples 1 to 6. Table 2shows the EMI shielding effectiveness at 15 GHz of the MWCNT/PMMAcomposite sheets prepared in Control Examples 7 to 12.

The results of the EMI shielding effectiveness at 15 GHz for the coatedPET film and the multiple-coated-PET film stacks prepared in Examples1-10 and Examples 11-20 are shown in Table 3 and Table 4, respectively.

TABLE 1 EMI shielding effectiveness at 15 GHz, MWCNT content, wt % dBControl Ex. 1 4.76 18.56 Control Ex. 2 0.25 1.82 Control Ex. 3 0.5 3.63Control Ex. 4 0.74 5.40 Control Ex. 5 0.99 4.65 Control Ex. 6 2.44 10.70

TABLE 2 EMI shielding effectiveness at 15 GHz, MWCNT content, wt % dBControl Ex. 7 4.76 20.29 Control Ex. 8 0.25 1.32 Control Ex. 9 0.5 1.98Control Ex. 10 0.74 3.17 Control Ex. 11 0.99 3.66 Control Ex. 12 2,4410.87

TABLE 3 EMI shielding effectiveness Number of films stacked at 15 GHz,dB Example 1 1 5.85 Example 2 2 8.84 Example 3 3 12.18 Example 4 4 15.09Example 5 5 17.03 Example 6 6 17.91 Example 7 7 21.91 Example 8 8 25.41Example 9 9 30.42 Example 10 10 41.98

TABLE 4 EMI shielding effectiveness Number of films stacked at 15 GHz,dB Example 11 1 4.11 Example 12 2 7.12 Example 13 3 8.97 Example 14 411.68 Example 15 5 15.15 Example 16 6 20.19 Example 17 7 22.95 Example18 8 27.28 Example 19 9 36.23 Example 20 10 45.36

The data shown in Tables 1 to 4 and FIGS. 1 to 4 indicate that thecoated PET film or the stacks of multiple sheets of the coated PET filmsprepared in Examples 1-20 of the present invention have better EMIshielding effectiveness in comparison with the MWCNT/PMMA sheets havingthe same MWCNT contents prepared in Control Examples 1-20. For examples41.98 dB of Example 10 (10-sheet stack) shown in Table 3 issignificantly higher than 18.56 dB of Control Example 1 (MWCNT content4.76 wt %) shown in Table 1; 1703 dB of Example 5 (5-sheet stack) shownin Table 3 is significantly higher than 10.70 dB of Control Example 6(MWCNT content 2.44 wt %) shown in Table 1.

The EMI shielding effectiveness between 2-18 GHz for the EMI shieldingmaterials prepared in Control Examples 1-6, Control Examples 7-12,Examples 1-10 and Examples 11-20 are shown in FIGS. 1 to 4,respectively. It can be seen from FIGS. 1 and 3 or FIGS. 2 and 4 thatthe coated PET film or the stacks of multiple sheets of the coated PETfilms prepared in Examples 1-20 of the present invention have better EMIshielding effectiveness in comparison with the MWCNT/PMMA sheets havingthe same MWCNT contents prepared in Control Examples 1-20.

1. A method of preparing carbon nanotube/polymer composite havingelectromagnetic interference (EMI) shielding effectiveness, whichcomprises the following steps: a) preparing a polymer solutioncontaining 0.5-10 wt % of carbon nanotubes dispersed therein, based onthe weight of the polymer; b) coating the polymer solution containingcarbon nanotubes dispersed therein on a substrate and drying theresulting layer coated on the substrate; and c) stacking a plurality ofthe substrates prepared from step b), each of which has the dried layer;wherein said preparing in step a) comprises dissolving a polymer in anorganic solvent and dispersing carbon nanotubes in the resulting polymersolution to form said polymer solution containing carbon nanotubesdispersed therein, and wherein said substrate in step b) is a film ofpoly(ethylene terephthalate), polyimide, polyethylene, polypropylene, orpoly(vinyl chloride).
 2. The method as claimed in claim 1, wherein stepc) further comprises applying an adhesive on the substrates prior tosaid stacking so that the stacked substrates are bonded.
 3. The methodas claimed in claim 1, wherein the dried layer on the substrate preparedin step b) has a thickness of 0.05 mm to 1.0 mm.
 4. The method asclaimed in claim 2, wherein the dried layer on the substrate prepared instep b) has a thickness of 0.05 mm to 1.0 mm, and 2 to 100 sheets of thesubstrates are stacked in step c).
 5. The method as claimed in claim 1,wherein the carbon nanotubes are multi-walled carbon nanotubes.
 6. Themethod as claimed in claim 1, wherein said polymer is selected from thegroup consisting of poly(acrylic acid), poly(methacrylic acid),poly(methyl acrylate), poly(methyl methacrylate), soluble polyimide,soluble poly(amide imide), polyamide, polystyrene, soluble polyurethane,unsaturated polyester, poly(ether sulfone), soluble poly(ether imide),poly(vinyl ester), thermoplastic polyurethane, silicone, and epoxyresin.
 7. The method as claimed in claim 6, wherein said polymer ispoly(methyl methacrylate).
 8. The method as claimed in claim 7, whereinsaid poly(methyl methacrylate) is prepared by polymerizing methylmethacrylate in a solvent of N,N-Dimethylacetamide and in the presenceof an initiator of 2,2-azobisisobutyronitrile at 120° C.
 9. The methodas claimed in claim 1, wherein said substrate in step b) is apoly(ethylene terephthalate) film.
 10. The method as claimed in claim 1,wherein said substrate in step b) is a polypropylene film.
 11. Themethod as claimed in claim 1, wherein said substrate in step b) is apoly(vinyl chloride) film.
 12. A method of using a carbonnanotube/polymer composite to provide electromagnetic interference (EMI)shielding comprising preparing a carbon nanotube/polymer composite byusing the method as defined in claim 1; and enclosing an electronicdevice in the carbon nanotube/polymer composite.